RSC Advances

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View Article OnlineView Journal | View Issue

The removal mec

aState Environmental Protection Key Laborat

Control on Chemical Process, East China

Shanghai, 200237, China. E-mail: linsen@ebNational Engineering Research Center f

Resources, East China University of SciencecSchool of Environment Science and Tech

Shanghai, ChinadShanghai Institute of Pollution Control and

† Electronic supplementary informa10.1039/c9ra03374b

Cite this: RSC Adv., 2019, 9, 24760

Received 6th May 2019Accepted 30th July 2019

DOI: 10.1039/c9ra03374b

rsc.li/rsc-advances

24760 | RSC Adv., 2019, 9, 24760–247

hanism and performance oftetrabromobisphenol A with a novel multi-groupactivated carbon from recycling long-rootEichhornia crassipes plants†

Lili Liu,ad Xin Chen,a Zhiping Wang,c Xixi Wanga and Sen Lin *abd

Long-root Eichhornia crassipes has shown great potential in eutrophication treatments while the heavy

disposal of its plants limits its large-scale application. In this study, the adsorption of TBBPA by a novel

multi-group activated carbon (MGAC), prepared from the reaped long-root Eichhornia crassipes plants

has been investigated as a potential recycling and remediation technology. The MGAC showed great

adsorption performance for aqueous TBBPA in that the adsorption could arrive at equilibrium in 4 h and

the saturated adsorption capacities could reach up to 110.7, 110.5 and 75.50 mg g�1 at 20, 30 and 40 �C,respectively. Based on the analysis of adsorption processes, it was confirmed that p–p interaction and

hydrogen bonding were the major impetuses for the adsorption and the oxygen-containing functional

groups on the MGAC surface could facilitate the adsorption by either electron sharing or electron

transfer. In addition, the thermodynamic results showed that the adsorption was a spontaneous and

exothermic reaction. Futhermore, the MGAC could be regenerated easily by 5% NaOH solution and

retained over 50% of its initial capacities for TBBPA after 5 reprocessing cycles. These results indicate the

promising application of MGAC in the wastewater treatment for TBBPA removal and a resource recycling

method for the long-root Eichhornia crassipes plants.

1. Introduction

Tetrabromobisphenol A (TBBPA) has been used extensively asa reactive or additive ame retardant in paper, textiles andplastics products, and especially for electronic products toreduce the ammability over the past decades.1 Up to now,TBBPA has been detected in various environmental samples, i.e.water,2–4 sediment,5,6 leachate,7 etc. Besides TBBPA can passthrough food chains and drinking water into the body, causinghuge harm to human health owing to its endocrine disruptioneffects and possible carcinogenicity.8 Thus there have beengrowing efforts worldwide to remove TBBPA, including oxida-tion,9,10 reduction,11,12 pyrolysis13 and adsorption.14–20 Amongthem, adsorption by activated carbon has been one of the most

ory of Environmental Risk Assessment and

University of Science and Technology,

cust.edu.cn

or Integrated Utilization of Salt Lake

and Technology, Shanghai, China

nology, Shanghai Jiao Tong University,

Ecological Security, Shanghai, China

tion (ESI) available. See DOI:

69

extensively investigated methods due to its cheapness, effec-tiveness and convenience for subsequent operation.

At the same time, the high cost of raw materials (coconutshell, wood, coal, etc.) for commercial activated carbon leads tothe high price of large-scale wastewater treatment using acti-vated carbon. Therefore, the preparation of activated carbonfrom plant materials in natural environment has the advantagesof low cost, environmental friendliness and rich structures.Among them, long-root Eichhornia crassipes with the charac-teristics of rapid growth and reproduction, wide distributionand easy access, has been planted to solve the eutrophication inDianchi Lake in China dependent on its ultralong root, whilehow to dispose these ripe plants is still a great challenge.Therefore, it's essential and of great environmental signicanceto explore available approaches for the recycle of waste long-root Eichhornia crassipes plants for the practical applicabilityof this eutrophication treatment technology. According to thecharacteristics of the long-root Eichhornia crassipes, it would becomparatively protable to transform and recycle them into thehigh-performance activated carbon for environmental contam-inants. At present, most of the studies were focused on theadsorption of heavy metals by long-root Eichhornia crassipesactivated carbon, while there are few reports on the adsorptionof organic pollutants, which limited its further promotion.21–23

Therefore, it is necessary to increase the research on the

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adsorption mechanism of organic matters. In addition, it isextremely necessary to understand the underlying adsorptionprocesses and mechanisms for the adsorption to furtherimproving the practical performance of adsorbents.

In this study, a novel multi-group activated carbon (MGAC)was prepared from the long-root Eichhornia crassipes to inves-tigate the adsorption performance and specic mechanism foraqueous TBBPA in the rst time. Aer the preparation, thephysicochemical characteristics of the MGAC were determined.Then the effects of different exogenous factors on adsorptionwere explored to deduce the correlative impetuses, includinginitial pH, ionic strength, etc. In addition, the adsorptiondynamics and thermodynamics were described by multifariousmodels to analyze adsorption performance and mechanism.Finally, the regeneration and recycling of the MGAC were alsoinvestigated. Based on these results, the in-depth under-standing of the adsorption mechanism and demonstration ofthe adsorption, regeneration and reuse of the MGAC was ob-tained and improved. It further indicated the feasibility of thepractical application and recycling utilization of long-rootEichhornia crassipes plants.

2. Materials and methods2.1 Chemicals

TBBPA used for adsorption was purchased from J&K ScienticCo., Ltd (Shanghai, China) with the purity over 98.0%. The otherchemicals used in this work were all of analytically reagentgrade and no further purication was performed prior to use.The TBBPA stock solution was prepared by dissolving 100 mgTBBPA into 100 ml 0.1% (m/m) NaOH solution. This solutionwas further diluted with deionized water to the concentrationrequired for the following experiments. The long-root Eichhor-nia crassipes plants used to prepare the MGAC were reaped fromthe Dianchi Lake, Yunnan, China. In all experiments, the initialpH values of the solutions were adjusted by using 1%HCl or 1%NaOH solution.

2.2 Preparation of the MGAC

10.0 g powder of long-root Eichhornia crassipes, which had beendried and grinded, mixed with 30 ml 25% KOH solution understirring for primary carbonization. Then 100 ml deionized waterwas added to impregnate the powder for 12 h. Aerwards, thepowder was calcined at 600 �C for 1 h with nitrogen protectionaer dried at 100 �C. Finally, the MGAC could be collected bywash with 5% HCl to activate the calcined product.

2.3 Adsorption experiments

For a typical batch experiment, 15 mg MGAC was added to100 ml TBBPA work solution with selected concentration ata given pH in a 250 ml ask. The ask was then placed ina temperature-controlled incubator shaker (YRH-300, YaoshiInstrument, China) at 175 rpm at a given temperature. Thesupernatant was sampled at specied time intervals and lteredthrough a 0.22 mm lter for analysis. The adsorption capacitywas calculated according to eqn (1).

This journal is © The Royal Society of Chemistry 2019

qe ¼ (C0 � Ce) � V/m (1)

where C0 (mg L�1) and Ce (mg L�1) were the initial and equi-librium concentrations of the target contaminant, respectively.And qe (mg g�1) was the adsorption capacity of the MGAC atequilibrium. V (L) was the contaminant solution volume and m(g) was the mass of the MGAC.

All experiments were performed in triplicate and the varia-tions between parallel experiments were less than 5%. Allglassware used were cleansed via sonication at 40 kHz for30 min and then dried out before use.

2.4 Adsorption kinetics

The adsorption kinetic models were used to describe theadsorption process and the effect of contact time. 15 mg MGACmixed with 100 ml TBBPA solutions of concentrations 5, 10 and20 mg L�1 in 250 ml ask at pH 9.0, 175 rpm and 30 �C. Thesupernatant was sampled at specied time intervals and lteredthrough a 0.22 mm lter for analysis. All experiments wereperformed in triplicate and the variations between parallelexperiments were less than 5%.

The pseudo-rst-order and pseudo-second-order kineticmodels can be expressed as (2) and (3), respectively:

qt ¼ qe(1 � e�k1t) (2)

qt ¼ k2qe2t/(1 + k2qet) (3)

where qt (mg g�1) is the adsorbed amount at time t (h), qe (mgg�1) is the adsorbed amount at equilibrium, k1 (h

�1), k2 (g (mgh)�1) are the pseudo-rst-order and pseudo-second-order rateconstant, respectively.

The intra-particle diffusion equation is:

qt ¼ kipt1/2 + C (4)

where kip [mg (g h1/2)�1] is the rate constant in the intra-particlediffusion equation. The boundary layer thickness is determinedby C. If the plot of qt (mg g�1) vs. t1/2 (h1/2) is a straight line andpasses through the origin, the sole factor controlling theadsorption ratio is intra-particle diffusion.24

2.5 Adsorption isotherms

The adsorption isotherm models were used to determine theadsorption mechanism and describe how TBBPA moleculesinteract with the MGAC. 100 ml TBBPA solutions of veconcentrations, i.e., 2, 5, 10, 20, 30 mg L�1, were prepared andused in adsorption isotherm experiments. The supernatant wassampled at specied time intervals and ltered through a 0.22mm lter for analysis. All experiments were performed in trip-licate and the variations between parallel experiments were lessthan 5%.

Langmuir isotherm assumes that the adsorbate is uniformlyadsorbed on the surface of the adsorbent. The adsorptionprocess is monolayer and the adsorbate does not move on thesurface.

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Ce/Qe ¼ 1/(QmKL) + Ce/Qm (5)

where Qe (mg g�1) is the equilibrium adsorbance, Ce (mg L�1) isthe equilibrium concentration of TBBPA in the solution, Qm (mgg�1) is the saturation adsorption capacity, KL (L mg�1) is theLangmuir model constant.

The Freundlich models indicates that multilayer adsorptionis carried out on a heterogeneous surface, which was expressedby eqn (6).

ln Qe ¼ ln KF þ 1

nln Ce (6)

where Qe (mg g�1) is the equilibrium adsorbance, Ce (mg L�1) isthe equilibrium concentration of TBBPA in the solution, Qm (mgg�1) is the saturation adsorption capacity, KF and n are theFreundlich model constants.

The Temkin model is mainly used to describe the chemicaladsorption process. It is considered that the adsorption heatvaries linearly with temperature. The equation is as follows:

Qe ¼ B1 ln KT + B1 ln Ce (7)

where Qe (mg g�1) is the equilibrium adsorbance, Ce (mg L�1) isthe equilibrium concentration of TBBPA in the solution, B1 isthe reaction heat of adsorption, KT (L mg�1) is the Temkinmodel constants.

The Dubinin–Radushkevich (D–R) model equation is asfollows:

ln Qe ¼ ln Qm � k32 (8)

3 ¼ RT ln(1 + 1/Ce) (9)

where Qe (mg g�1) is the equilibrium adsorbance, Ce (mg L�1) isthe equilibrium concentration of TBBPA in the solution, k (mol2

J�2) is a constant that corresponds to the adsorption energy, 3 isthe Polanyi potential, R is the ideal gas constant [8.314 J (molK)�1], Qm (mg g�1) is the maximum adsorption capacity.

In addition, the adsorption thermodynamics parametersincluding standard enthalpy (DH0, kJ mol�1), standard entropy(DS0, J (mol K)�1) and Gibbs free energy (DG0, kJ mol�1) werecalculated according to eqn (10) and (11).

lnK ¼ DS0

R� DH0

RT(10)

DG0 ¼ DH0 � TDS0 (11)

where K (L g�1) is the adsorption equilibrium constant, R (8.314J (mol K)�1) is the universal gas constant, T is the temperature(K).

2.6 Regeneration

1.0 g of the used MGAC mixed with 30 ml 5% NaOH solution ina 150 ml ask and then the ask was placed in incubator shakerat 30 �C and 175 rpm for 2 h. At last the spent MGACwas washedwith deionized water until the solution pH reached 6.0–7.0.Aer the regeneration, the remaining TBBPA in the solution was

24762 | RSC Adv., 2019, 9, 24760–24769

measured, and the regenerated MGAC was applied for thesubsequent adsorption cycles.

2.7 Characterization methods

The TEM and SEM images of the MGAC were examined on antransmission electron microscope (JEM-2100, JEOL, Japan) witha high voltage of 200 kV and an scanning electron microscopy(JSM-6360LV, JEOL, Japan) with 5.0 kV, respectively. The specicsurface area was calculated by the Brunauer–Emmett–Teller(BET) method and the pore volume and pore size distributionwere estimated based on Barrett–Joyner–Halenda (BJH) model.The X-ray photoelectron spectroscopy (XPS) spectrum of thisadsorbent was obtained by an XPS instrument (ESCALAB 250Xi,Thermo Scientic, USA) with the Al-KR as the excitation source.The zeta potentials of the MGAC were determined by a zetapotential instrument (Nano ZS, Malvern, UK) following themethod of Wang et al.25 Fourier-transform infrared (FTIR)spectra of the MGAC before and aer adsorption were recordedfrom samples in the wavenumber range of 4000–400 cm�1 inKBr pellets on a FTIR spectrometer (NICOLET 6700, Thermo-sher, USA). The concentrations of TBBPA in solutions weredetermined by a high performance liquid chromatography(HPLC) (LC-20AT, Shimadzu, Japan) equipped with an UVdetector and a C18 reverse-phase column (Inertsil ODS)(250 mm � 4.6 mm i.d., particle size 5 mm) using methanol/water (80 : 20 (v/v)) mobile phase. The pH values of solutionswere measured with a pH meter (HQ30d, HACH, USA) aercalibration.

3. Results and discussion3.1 Characterization of the MGAC

In this study, multiple measurements were applied to charac-terize the physicochemical properties of the prepared MGAC.Firstly, the morphology and microstructure of the MGAC weredemonstrated by the TEM and SEM images (Fig. 1). The TEMobservation of the MGAC indicated that the carbon hadsignicantly hierarchical porous structures with honeycomb-like shape (Fig. 1a). And the SEM (Fig. 1b) observation pre-sented a rough and irregular surface with pores on the MGAC.According to the BET result, it might be related to the porosity ofthe adsorbent, thus resulted in a high specic surface area26,27

reaching than 874.3 m2 g�1. The pore size distribution curve(Fig. 1c) exhibited in the category of mesopore, with the porevolume and average size being at 0.38 cm3 g�1 and 6.96 nm,respectively. In addition, the isoelectric point of the MGAC wasidentied from variation trend of the zeta potential as a func-tion of pH values. As shown in Fig. 1d, the zeta potentialdecreased with the increasing pH, from which the isoelectricpoint was determined as 3.3, indicating the MGAC surface waselectronegative in non-acid condition.

To further understand the functional groups on the MGACsurface, the wide XPS spectrum of the MGAC was surveyed(Fig. S1†), implying the oxygen, carbon and nitrogen were thedominant elements. To further explore the surface carbon andoxygen functional components, the high-resolution spectrum of

This journal is © The Royal Society of Chemistry 2019

Fig. 1 The SEM (a) and TEM (b) images of the MGAC; the pore size distribution (c) of the MGAC; the zeta potential of the MGAC as a function ofpH (d).

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C1s and O1s were deconvoluted by XPSPEAK41 soware and theresults were plotted in Fig. 2. And the C1s peak could bedecomposed into four individual peaks at 284.6, 285.4, 286.6

Fig. 2 High resolution XPS spectra of C1s (a) and O1s (b) on the MGAC

This journal is © The Royal Society of Chemistry 2019

and 289.2 eV (Fig. 2a), which were responsible for C–C inaromatic rings, C–C]O, C–O and O–C]O, respectively.28,29 Inthe case of oxygen in Fig. 2b, there were also two distinct oxygen

surface.

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Fig. 3 Effect of initial pH on TBBPA adsorption using the MGAC(TBBPA ¼ 10 mg L�1, MGAC ¼ 150 mg L�1, T ¼ 30 �C, t ¼ 10 h).

Fig. 4 Effect of ionic strength on TBBPA adsorption using the MGAC(TBBPA ¼ 10 mg L�1, MGAC ¼ 150 mg L�1, T ¼ 30 �C, pH ¼ 9.0, t ¼ 10h).

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bands at 532.4, and 533.6 eV, which were ascribed to C–O andC–OH, respectively, consistent with previous work.14 Theproportions of functional groups both in C1s and O1s peakindicated that there were abundant oxygen-containing func-tional groups on theMGAC surface, affording great potential forthe adsorption (Table S1†).

3.2 Effects of environmental factors on the adsorption ofTBBPA

3.2.1 Initial pH. pH is one of the vital factors affecting theadsorption performance, which determines the surface chargeon the adsorbent surface as well as the existing speciation ofadsorbate in solution. The experiments were conducted withinitial pH value varying in the range of 9.0–12.0, in view of theinsolubility of TBBPA in non-alkaline solution.30 As shown inFig. 3, TBBPA adsorbed on the MGAC decreased from 52.1 mgg�1 to 8.0 mg g�1 with the increase of initial pH in solution from9.0 to 12.0, indicating the alkaline condition unfavorable for theadsorption. According to the distribution of molecular and

24764 | RSC Adv., 2019, 9, 24760–24769

anionic forms as a function of pH, illustrated in Fig. S2,† TBBPAwas mostly ionized to mono or divalent anions in the alkalinecondition.18 As the pH value exceeded 9.0, these functionalgroups on the MGAC surface were deprotonated, such as–COO�, –O�, etc., resulting in the decrease of the adsorptioncapacity due to the enhanced electrostatic repulsion betweenthe MGAC surface and TBBPA. In addition, attributed to thereduction of hydrogen bonding in strongly basic solution,which was prevented by the enhanced charge repulsion, TBBPAadsorption decreased severely when pH increased from 10.0 to12.0.

3.2.2 Ionic strength. In this study, the effects of ionicstrength on TBBPA adsorption using the MGAC was performedwith the concentrations of NaCl at the range of 0–50 g L�1. Theadsorption capacity decreased slightly with NaCl concentrationin the range of 0–3 g L�1 (Fig. 4), due to that the electrostaticrepulsion would be reinforced under a small amount of NaCl insolution. On the contrary, the adsorption capacity of TBBPAincreased from 48.4 mg g�1 to 66.0 mg g�1 with the increase ofNaCl concentration in 3–50 mg L�1. According to the results ofAmrit et al.,31 the increased adsorption capacity might comefrom the p–p interaction between TBBPA and the MGAC, whichcould be effectively enhanced by stronger ionic strength. Inaddition, Na+ bridged with negatively charged surface groupsmight promote the adsorption resulting from offsetting thenegative effects of competing adsorption sites.32

3.2.3 Humic acid.Humic acid (HA) is ubiquitous in naturalwaters derived by the microbial degradation of dead plants,which is always concomitant with TBPPA.33 Hence, it's full ofimportance to investigate the effect of the coexistence of HA onTBBPA adsorption. According to results of TBBPA adsorption atdifferent concentrations of HA (Fig. 5), the adsorption capacitydecreased signicantly with HA concentration less than10 mg L�1 while the adsorbed TBBPA was enhanced as HAconcentration continued to increase. The basic structure of HAis aromatic and alicyclic ring, connected with carboxyl,carbonyl, quinonyl, hydroxyl and methoxy groups.34 All thesestructure can form the p–p interaction, hydrogen bonding withMGAC, which could attributed to the competitive adsorption ofHA and TBBPA on adsorbents thus reduce the removal effi-ciency of TBBPA.35,36 The slight increase in adsorption at highconcentration of HA was presumably due to the adsorption offree TBBPA to HA which was sorbed to the MGAC surface.37

3.3 Adsorption kinetics

The adsorption kinetics of TBBPA using the MGAC was con-ducted under the initial concentrations of 5, 10 and 20 mg L�1.As shown in Fig. 6, it could be seen that the TBBPA adsorped onthe MGAC surface all increased quickly in the rst 2 h owing tothe rich availability of active sites, and then the adsorption rateslowed down until the adsorption equilibrium reached in 4 h. Inorder to further analyze the adsorption processes, the pseudo-rst-order and pseudo-second-order kinetic models were bothused to t the adsorption kinetic data shown in Fig. 6a. It couldbeen seen that the adsorption was all better described by thepseudo-second-order model for different initial concentrations,

This journal is © The Royal Society of Chemistry 2019

Fig. 5 Effect of HA on TBBPA adsorption using the MGAC (TBBPA¼ 10 mg L�1, MGAC¼ 150 mg L�1, HA¼ 0, 5, 10, 20, 50 mg L�1, T ¼ 30 �C, pH¼ 9.0, t ¼ 10 h).

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which was veried by the tting coefficients listed in Table 1higher than 0.96. According to the hypothesis of the pseudo-second-order model, it indicated that the adsorption rate wascontrolled by chemical interaction, and there might be electronexchange or covalent bond between adsorbents and adsorbates.Besides the adsorption capacity was proportional to the func-tional groups involving p–p bonding and hydrogen bondinginteractions on the absorbent, consistent with the studies ofZhang et al.18

The plots of qt vs. t1/2 at different initial concentrations are

shown in Fig. 6b. It could been seen that the plots are not linearand do not pass through the origin, which demonstrated thatthe adsorption was a complicated process including multiplesteps.38 Fig. 6b clearly shown that the adsorption could bedivided into three steps. The rst step was the instantaneous

Fig. 6 The adsorption kinetics of TBBPA on the MGAC (TBBPA ¼ 5, 10 a

This journal is © The Royal Society of Chemistry 2019

adsorption or external surface adsorption in the range of 0–0.5 hand approximately 50% of the adsorption capacities nished inthis step mainly relying on the high concentration of TBBPA asthe adsorption impetus.39 The second step was controlled by theintra-particle diffusion and the curve represented gradualadsorption, where TBBPA was removed by the internal sites.24 Inthe nal equilibrium stage, the adsorption growth rate wasalmost constant and TBBPA concentration kept at a low level,indicating the adsorption equilibrium arriving.40

3.4 Adsorption thermodynamics

To further understand the adsorption mechanisms of TBBPA bythe MGAC, adsorption isotherms were determined at tempera-ture being at 20, 30 and 40 �C with various models of Langmuir,

nd 20 mg L�1, MGAC ¼ 150 mg L�1, T ¼ 30 �C, pH ¼ 9.0, t ¼ 10 h).

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Table 1 Kinetic fitting parameters for TBBPA adsorption on the MGAC

C0 (mg L�1)

Pseudo-rst-order Pseudo-second-order

k1 (h�1) qe (mg g�1) R2 k2 g (mg h)�1 qe (mg g�1) R2

5 2.164 24.20 0.9567 0.137 25.48 0.997410 1.329 50.93 0.9800 0.033 55.70 0.994020 2.223 69.84 0.9559 0.041 74.57 0.9623

Intra-particle diffusion model

C0 (mg L�1)

First step Second step Third step

Kip 1 (mg g�1 h�0.5) C1 (mg g�1) R2 Kip 2 (mg g�1 h�0.5) C2 (mg g�1) R2 Kip 3 (mg g�1 h�0.5) C3 (mg g�1) R2

5 2.5451 �0.4788 0.9839 5.8897 12.5903 0.9259 0.7023 22.5618 0.727410 42.1574 �2.0828 0.9085 15.1477 19.8061 0.9766 1.9223 46.2472 0.755520 72.6213 �4.9208 0.8346 14.6827 41.2785 0.9382 1.5370 66.4384 0.8304

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Freundlich, Temkin and D–R equations, respectively (Fig. 7).The corresponding models and tting parameters were given inTable 2. Among the different models, the Langmuir model, with

Fig. 7 Adsorption isotherms of TBBPA on MGAC in different temperaturand (d) D–R equations (TBBPA ¼ 2, 5, 10, 20 and 30 mg L�1, MGAC ¼ 1

24766 | RSC Adv., 2019, 9, 24760–24769

the higher correlation coefficient values (R2 > 0.981), coulddescribe the adsorption better than the other isotherm models,indicating the existence of monolayer adsorption in the TBBPA

e as well as modeling using the (a) Langmuir, (b) Freundlich, (c) Temkin50 mg L�1, T ¼ 20, 30 and 40 �C, pH ¼ 9.0, t ¼ 10 h).

This journal is © The Royal Society of Chemistry 2019

Table 2 Isotherm parameters for TBBPA adsorption on MGAC

T(�C)

Langmuir Freundlich Temkin D–R

Qm (mg g�1) KL (L mg�1) R2 KF n R2 B1 KT R2 Qm k R2

20 110.7 0.3919 0.993 25.768 1.839 0.907 22.750 4.422 0.978 71.0 1.012 0.83030 110.2 0.3070 0.981 21.979 1.726 0.902 22.969 3.423 0.962 66.5 1.250 0.79240 75.5 0.6156 0.993 26.618 2.691 0.969 12.794 12.931 0.990 53.7 0.410 0.807

Table 3 Thermodynamic parameters for TBBPA adsorption on theMGAC at different temperatures

T(�C)

DG0 (kJmol�1)

DH0 (kJmol�1)

DS0 (Jmol�1 K�1)

20 �18.79 �4.97 47.1630 �19.2640 �19.76

Fig. 8 FTIR spectra of the MGAC (a) and the MGAC with TBBPAadsorbed (b).

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removal process. According to the tting results, the saturatedadsorption capacity of the MGAC for TBBPA came up to110.7 mg g�1, which was signicantly superior to commoncommercial activated carbons (Fig. S3†). Moreover, the n valuesof Freundlich model ranged from 1.726 to 2.691 indicated thatthis adsorption process was feasible and all adsorptionisotherms were nonlinear,25 which could be attributed to the

Table 4 The peaks of FTIR spectra corresponding to functional groups

Peak (cm�1) Characteristic vibration

3421 O–H vibration2970 O–H vibration2922 C–H stretching vibratio1581 Stretching vibrations of1448 Stretching vibrations of1385 O–H bending vibration1045 C–O stretching vibratio877 C]C vibration

This journal is © The Royal Society of Chemistry 2019

various functional groups on the MGAC and the differentdominant interactions like hydrogen bonding in the adsorptionprocess.

In addition, the thermodynamics parameters including DG0,DH0 and DS0 were also calculated and listed in Table 3. Theresults showed that DG0 was negative at different temperatures,indicating that the TBBPA adsorption using the MGAC wasa thermodynamically feasible and spontaneous process. Thenegative DH0 illustrated that the adsorption was an exothermicprocess, which would present preferable performance withlower temperature.

3.5 Adsorption mechanism of the MGAC for TBBPA

According to previous studies, diversied adsorption impetusesto remove TBBPA using activated carbons had been proposed,including electrostatic interactions, hydrogen bonding, p–p

interactions, hydrophobic effect.32 To further understand theadsorption mechanisms of TBBPA onto the MGAC surface, theFTIR spectra of the MGAC before and aer adsorption werecompared (Fig. 8). In addition, the peaks corresponding func-tional groups were demonstrated which certied the existenceof multifarious oxygen-containing groups on the MGAC surface(Table 4). It is worth to highlight the change of the band at1045 cm�1 aer adsorption, which could be ascribed to thecombination between TBBPA and –OH groups on the MGACsurface via hydrogen bonding.17 The disappearance of band at2922 cm�1 and shi of band at 1581 cm�1 to 1564 cm�1 aeradsorption could be attributed to the p–p interactions betweenTBBPA and the MGAC resulting in the variations of electrondensity and on the MGAC surface.18

Based on these results, the possible adsorption processes ofTBBPA onto the MGAC surface were further proposed (Fig. 9). Itwas deduced that the p–p interactions probably playeda signicant role in the adsorption due to the benzene rings and

Functional groups

–OH41

–OH42

n –CHx17

aromatic rings Aromatic rings43

aromatic rings Aromatic rings43

–OH17

n C]O44

C]C17

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Fig. 9 Schematic diagram for the adsorption of TBBPA using the MGAC.

Fig. 10 Regeneration of MGAC by 5% NaOH (TBBPA ¼ 10 mg L�1,MGAC ¼ 150 mg L�1, T ¼ 30 �C, pH ¼ 9.0, t ¼ 10 h).

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–OH groups on the MGAC surface as the electron-donatingfunctional groups might enhance the p-donating strength ofthe aromatic rings of TBBPA. In addition, hydrogen bondingalso generated signicant inuence on the adsorption for theMGAC could act as hydrogen bond donors dependent on the–OH groups and aromatic rings on the surface, resulting ina better adsorption capacity. Besides, there may be electronexchange or covalent bond between adsorbents and adsorbatesaccording to the results of kinetic model analysis.

3.6 Regeneration of MGAC

In order to extend the lifetime of the MGAC, the usedMGAC wasregenerated using NaOH solution as the eluent and then sub-jected to the TBBPA adsorption again. As shown in Fig. 10, thespent MGAC was successfully regenerated by alkaline andretained over 50% of its initial capacity for TBBPA aer 5

24768 | RSC Adv., 2019, 9, 24760–24769

successive adsorption–regeneration cycles, which could beattributed to a decrease in the number of adsorption sites. Theresults nevertheless proved that the MGAC based the long-rootEichhornia crassipes could be used repeatedly in an adsorption–desorption cycle, which was an essential advantage with regardto the practical applications. In addition, many regenerativemethods have been developed recently, such as pyrolysis, whichis versatile because it can decompose various organic contam-inants. Studies have conrmed that TBBPA can be pyrolyzed at280–900 �C,45 which provides a new efficient strategy for theMGAC regeneration in the future application.

4. Conclusion

In this study, the MGAC prepared from the waste long-rootEichhornia crassipes plants, had abundant functional groupson its surface, which could effectively remove TBBPA, indicatingthat it had good adsorption performance for organic pollutants.Moreover, it was conrmed that p–p interaction and hydrogenbonding were the major impetuses for the adsorption on thebasis of multiple characterization and experimental analysis.The in-depth understanding of the adsorption properties andmechanism will offer valuable counsel for the application of theMGAC in the wastewater treatment and provide a new way forthe resource utilization of long-root Eichhornia crassipe in thefuture.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

This work was sponsored by the National Natural ScienceFoundation of China (41771513, 51704121, 41001316,51108262), Major Science and Technology Program for WaterPollution Control and Treatment in China (2017ZX07202006)

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and Shanghai Pujiang Program (16PJ1404800), National KeyR&D Program of China (2018YFC1901000).

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